This project has been designed to determine the blood damage encountered in short and long term heart-lung bypass due to microporous membrane oxygenators. Frank and occult damage to formed elements of the blood are measured by unambiguous experiments. Comparison with results for solid membrane oxygenators is made. These data are interpreted with help of a model of transport processes at the microporous membrane-blood interface. The studies are intended to put the evaluation of microporous surfaces on a firm experimental and analytical basis.